Question

I have a List<Task<bool>> that I want to enumerate in parallel finding the first task to complete with a result of true and not waiting for or observe exceptions on any of the other tasks still pending.

var tasks = new List<Task<bool>>
{ 
    Task.Delay(2000).ContinueWith(x => false), 
    Task.Delay(0).ContinueWith(x => true), 
};

I have tried to use PLINQ to do something like:

var task = tasks.AsParallel().FirstOrDefault(t => t.Result);

Which executes in parallel, but doesn’t return as soon as it finds a satisfying result. because accessing the Result property is blocking. In order for this to work using PLINQ, I’d have to write this aweful statement:

var cts = new CancellationTokenSource();
var task = tasks.AsParallel()
    .FirstOrDefault(t =>
    {
        try 
        { 
            t.Wait(cts.Token);
            if (t.Result)
            {
                cts.Cancel();
            }

            return t.Result;
        } 
        catch (OperationCanceledException) 
        { 
            return false;
        }
    } );

I’ve written up an extension method that yields tasks as they complete like so.

public static class Exts
{
    public static IEnumerable<Task<T>> InCompletionOrder<T>(this IEnumerable<Task<T>> source)
    {
        var tasks = source.ToList();
        while (tasks.Any())
        {
            var t = Task.WhenAny(tasks);
            yield return t.Result;
            tasks.Remove(t.Result);
        }
    }
}

// and run like so
var task = tasks.InCompletionOrder().FirstOrDefault(t => t.Result);

But it feels like this is something common enough that there is a better way. Suggestions?

Answer 1

Maybe something like this?

var tcs = new TaskCompletionSource<Task<bool>>();

foreach (var task in tasks)
{
    task.ContinueWith((t, state) =>
    {
        if (t.Result)
        {
            ((TaskCompletionSource<Task<bool>>)state).TrySetResult(t);
        }
    },
        tcs,
        TaskContinuationOptions.OnlyOnRanToCompletion |
        TaskContinuationOptions.ExecuteSynchronously);
}

var firstTaskToComplete = tcs.Task;